1. Field of the Invention
The present invention relates to an improved process for the production of 2,2-(4,4' dihydroxydiphenylpropane, and particularly to a process for the production of 2,2-(4,4'-dihydroxydiphenyl)propane having a high degree of purity.
2. Description of the Prior Art
2,2-(4,4'-Dihydroxydiphenyl)propane, commonly known as "bisphenol A," is the condensation product of two molecules of phenol with one molecule of acetone. The condensation is catalyzed by acidic substances. In practice, bisphenol A is prepared by reaction of acetone with excess phenol in the presence of strong acids and optionally in the presence of accelerators. Usually, sulphuric acid and in particular hydrogen chloride are used as strong acids while mercaptans or mercapto acids are used as accelerators. The reaction yields a crude mixture containing various undesirable by-products in addition to bisphenol A, unreacted phenol and acetone, the acidic catalyst, any activator, and the reaction water. These undesirable by-products include, for example, isomers of bisphenol A having different properties, particularly 2,2-(2,4'-dihydroxydiphenyl)propane and 2,2-(2,2'-dihydroxydiphenyl)propane, complex products such as the so-called "co-dimer" (2,2,4-trimethyl-4-p-hydroxyphenylchroman), condensation products such as trisphenol, even higher condensation products in the form of tarry and high-boiling substances, and decomposition products.
The presence of these by-products involves considerable disadvantages both because of their tendency to remain in the final product and to cause undesirable discolouring of the final product. These tendencies are often pronounced to an extent that the use of the final product is prejudiced, and not only in cases where high purity is required. Moreover, the presence of the decomposition products inhibits some reactions in which bisphenol A usually may be used, for example in the preparation of polycarbonates. The importance of obtaining bisphenol A of high purity therefore seems obvious, and numerous methods have been proposed in the art for this purpose.
According to one of these methods (U.S. Pat. No. 2,191,831), the crude reaction mixture is treated so as to separate bisphenol A directly in crystalline form. However, this is a very complicated and expensive method not only for the numerous washes to which the crystalline bisphenol A must be subjected, but also for the rigorous and careful regulation of the conditions necessary for satisfactory crystallization. Moreover, only rather low yields of bisphenol A are attained.
According to another method (French Pat. No. 1,374,477) the crude condensation mixture is subjected to fractional distillation in order to separate at first the unreacted products and then bisphenol A, which is finally purified by extraction or recrystallization. However, this procedure involves considerable losses of bisphenol A. Additionally, in order to obtain bisphenol A having high purity, repeated extraction or recrystallization is necessary. Thus, even if the distillation is carried out under precautionary conditions, this process involves decomposition of the bisphenol A so that new undesired contaminants are formed, particularly products that give unsatisfactory colour characteristics. Furthermore, complete separation by distillation is often rather difficult and unattainable in practice. This is due to the fact that some components, particularly the isomers of bisphenol A have boiling points very close to bisphenol A.